A water absorbent resin, which has a characteristic of absorbing an aqueous solution whose amount is as many as several times to several hundred times its own weight, is extensively used for, for example, sanitary materials such as a disposable diaper, a sanitary napkin, and an adult incontinence product, and a soil water retention agent. The water absorbent resin is produced and consumed in a large amount. Such a water absorbent resin (also referred to as a high water absorbent resin or a water absorbent polymer) is described in, for example, the Japanese Industrial Standards (JIS) K7223-1996. Further, the water absorbent resin is also introduced in many commercially-available reference books and thus has been publicly known.
Especially for use in sanitary materials such as a disposable diaper, a sanitary napkin, and an adult incontinence product, in order to make products thinner, a tendency has recently been shown toward use of a water absorbent resin in a larger amount and use of a pulp fiber in a smaller amount. This requires the water absorbent resin to carry out conventional functions of pulp to permeate and diffuse a liquid permeability in an absorbent core. An absorbency against pressure and liquid permeability of the water absorbent resin have been proposed as publicly-known indicators to evaluate superiority or inferiority of such functions, and the absorbency against pressure and the liquid permeability are desired to be high. In contrast, such a tendency toward thinner products leads to an increase in amount of the water absorbent resin used for each sanitary material. Accordingly, a demand for a low-cost water absorbent resin increases.
A water absorbent resin is commonly produced by drying a hydrogel polymer obtained by polymerizing an aqueous solution containing a hydrophilic monomer and a cross-linking agent, and carrying out a surface treatment with respect to the dried hydrogel polymer. For example, poly(meth)acrylic acid (salt) is well known as the hydrophilic monomer. A hydrogel polymer obtained by polymerizing the hydrophilic monomer is obtained as a mass or an agglomerate of hydrogel particles. Normally, the hydrogel polymer is roughly crushed (roughly disintegrated) to have a particle size of approximately 1 mm to 10 mm by use of a crusher such as a kneader or a meat chopper. Then, the hydrogel polymer thus roughly crushed (roughly disintegrated) is dried to have a solid content of approximately 95 wt %.
In a pulverizing step after the drying, the hydrogel polymer thus dried is pulverized into particles by use of a pulverizer so that the particles have a weight average particle size of not less than 150 μm and not more than 850 μm. A particulate water absorbent resin is thus obtained. In this case, particles whose size is beyond a desired particle diameter (particle size) range are also contained in the particulate water absorbent resin. Therefore, the polymer thus pulverized after the drying is sieved by use of a classifier so as to have a particle diameter falling within the desired particle diameter range. Though there is a difference between purposes of use, a particulate water absorbent resin having a particle size falling within a range of not less than 150 μm to less than 850 μm is normally preferably used for a sanitary material.
The particulate water absorbent resin is subjected to a surface treatment step and then obtains properties such as an absorbency against pressure and liquid permeability which are desirable for a sanitary material and the like. Normally, the surface treatment step refers to (i) a step (a surface cross-linking step) of providing a highly cross-linked layer in a vicinity of a surface of the particulate water absorbent resin by causing light or heat to react the particulate water absorbent resin with a surface cross-linking agent or a polymerizable monomer or (ii) a step (an adding step) of providing an additive layer in the vicinity of the surface of the particulate water absorbent resin by adding, to the particulate water absorbent resin, functionality giving additives (surface treatment agents) such as a liquid permeability enhancer, a deodorant agent, a color protector, an antibacterial agent, and an antiblocking agent.
Note that in surface cross-linking a water absorbent resin which is composed mainly of poly(meth)acrylic acid (salt), a surface cross-linking agent contains a cross-linking agent containing a functional group which can react with a carboxyl group, or a polymerizable monomer.
Examples of a conventionally studied surface cross-linking technique include a technique related to use of surface cross-linking agents in combination (Patent Literature 1), a technique related to an apparatus which mixes a water absorbent resin and a surface cross-linking agent (Patent Literature 2), a technique related to a heating apparatus for reacting a water absorbent resin with a surface cross-linking agent (Patent Literature 3), a technique related to temperature increase control of a heating temperature for reacting a water absorbent resin with a surface cross-linking agent (Patent Literature 4), and a technique related to a surface cross-linking treatment carried out with respect to a water absorbent resin having a high water content (Patent Literature 5). Techniques are also known such that differently from common surface cross-linking, a water absorbent resin is reformed by heat application without using a surface cross-linking treatment (Patent Literatures 6 and 7).
Various surface cross-linking agents have also been proposed. Examples of such a surface cross-linking agent include an oxazoline (Patent Literature 8), a vinyl ether compound (Patent Literature 9), an epoxy compound (Patent Literature 10), an oxetane compound (Patent Literature 11), a polyhydric alcohol compound (Patent Literature 12), polyamidepolyamine-epihalohydrin adducts (Patent Literatures 13 and 14), a hydroxy acrylamide compound (Patent Literature 15), an oxazolidinone compound (Patent Literature 16), bis or poly-oxazolidinone compound (Patent Literature 17), 2-oxotetrahydro-1,3-oxazolidine compound (Patent Literature 18), and an alkylene carbonate compound (Patent Literature 19).
Further, techniques are also known for surface cross-linking by polymerizing a monomer (Patent Literatures 20 and 21) and for radical cross-linking by use of persulfate or the like (Patent Literature 22). Note that, since a radical cross-linking reaction requires retention of water contained in a water absorbent resin, Patent Literatures 21 and 22 describe that heat drying is carried out in an atmosphere of superheated steam.
A technique has also been proposed for using an additive in combination to mix a surface cross-linking agent with a water absorbent resin. Examples of the additive include water-soluble cations such as an aluminum salt (Patent Literatures 24 and 25), alkali (Patent Literature 26), and an organic acid and an inorganic acid (Patent Literature 27). A technique is also known for using a specific mixer for a mixer of a surface cross-liking agent with a water absorbent resin (Patent Literature 27).
Proposed examples of a technique for use in a heating step include a technique for carrying out surface cross-linking two times (Patent Literature 29), a technique for using a plurality of heating treatment apparatuses (Patent Literature 30), and techniques for preliminarily heating a water absorbent resin which has not been surface cross-linked (Patent Literatures 31 and 32). Besides Patent Literatures 20 and 21, techniques have also been proposed for using water vapor for a heat reaction in a step of heating a water absorbent resin with which a surface cross-linking agent that reacts with a carboxyl group is mixed (Patent Literatures 33 and 34).
As for a technique for mixing a particulate water absorbent resin and a surface treatment agent, several methods have been devised for controlling a movement of a particulate water absorbent resin in a mixer. Such a method is exemplified by a method for causing an air current caused by a stirring vane to control a movement of a particulate water absorbent resin which moves in a mixing vessel by.
For example, Patent Literature 28 describes that a speed at which a particulate water absorbent resin moves in a mixing vessel toward an outlet is changed by a direction of a stirring vane between a dispersion process and a mixing process of the particulate water absorbent resin.
Patent Literature 35 describes that mixing property is enhanced in a mixer by causing a stirring vane to adjust a ratio between particulate water absorbent resins moving in opposite directions: toward an outlet of the mixer and toward an inlet of the mixer. Patent Literature 35 also describes that, since a particulate water absorbent resin to which a surface treatment agent is adhered agglomerates and causes mixing property to deteriorate, a particulate water absorbent resin has kinetic energy enough to prevent agglomeration of the particulate water absorbent resin in a mixing process.
Patent Literature 35 also describes, as a mixer used to mix a particulate water absorbent resin and an additive, apparatuses such as a Patterson-Kelly mixer, a DRAIS turbulent mixer, a Lödige mixer, a Ruberg mixer, a screw mixer, a Pan mixer, a fluidized-bed mixer, an MTI mixer, and a Schugi mixer.
Patent Literature 2 describes a technique for using a water-repellent base material for an inner wall of a mixer to prevent adhesion of a particulate water absorbent resin and enhance mixing property. Patent Literature 2 also describes mixers used to mix the particulate water absorbent resin and a surface cross-linking agent such as Henschel Mixer (produced by Mitsui Miike Machinery Co., Ltd.), New Speed Mixer (produced by Okada Seiko Co., Ltd.), Heavy Duty Matrix (produced by Nara Machinery Co., Ltd.), and Turbulizer and Sand Turbo (which are produced by Hosokawa Micron Group).
As another method for controlling a movement of a particulate water absorbent resin in a mixing vessel, Patent Literature 36 describes that, in a case where a horizontal mixer, i.e., a mixer having stirring vanes whose revolving shafts are in a horizontal direction, is provided, in a vicinity of its outlet or between the stirring vanes, with a dam having whose height is not more than 50% of a revolving circumferential diameter, it is possible to (i) control time for which the particulate water absorbent resin resides in the mixing vessel and (ii) prevent the particulate water absorbent resin from passing straight through the mixer.
In contrast, a particulate water absorbent resin which is produced in a series of production processes of a water absorbent resin and is smaller than a desired particle size range is called fine powder. Especially fine powder whose particle size is less than 150 μm is unsuitable for use in a sanitary material since an absorbing article such as a diaper clogged with the fine powder may cause a deterioration in liquid permeability.
However, since disposal of fine powder causes problems of a yield reduction and a disposal cost, it is known that fine powder is appropriately collected and recycled. Examples of a known method for recycling fine powder include a method in which the fine powder is added to a monomer regulator, so as to be recycled (for example, see Patent Literature 37) and a method in which the fine powder is added to and mixed with a gel substance being polymerized or having been polymerized, so as to be recycled (for example, see Patent Literature 38). However, the method in which the fine powder is added to the monomer regulator or the gel substance as described above has a problem such that it is difficult to mix the fine powder and the monomer regulator or the gel substance.
As another method for collecting fine powder, a method is known in which fine powder is collected by adding a binding agent as an additive to the fine powder and binding the fine powder so that the bound fine powder has a particle size of not less than 150 μm (for example, see Patent Literature 39).
An aqueous solution or a fine particle aqueous dispersion liquid is commonly preferably usable as a binding agent for fine powder in view of efficiency, safety, production cost, and the like. In order to enhance mixing property of fine powder and a binding agent, there are known a method using hot water as a binding agent (Patent Literature 40) and a method using water vapor as a binding agent (Patent Literature 41). These Patent Literatures describe that a high-speed stirring mixer such as Turbulizer (produced by Hosokawa Micron Group), Lödige mixer (produced by Lödige), or Mortar Mixer (produced by Nishinihon Shikenki) is used to mix fine powder and a binding agent. A method is also known for binding fine powder by use of a specific mixer (Patent Literature 42).